The specific aims of the study are to charaterize the phenotypes and modes of inheritance of the genetic forms of hypodontia; to determine the intrafamilial variablity and delineate heterogeneity in hypodontia; to explore the linkages of hypodontia to marker loci; and to identify dental characteristics which may themselves be used as markers. Probands will be ascertained primarily from privately practicing orthodontists and pedodontists. The teeth of probands and relatives will be evaluated using clinical examinations, radiographs and satudy casts to determine whether or not teeth are congenitally missing, and if so, their location and number. tooth measurements in presumed unaffected parents of multiple affected sibs will be made to determine whether, they have the gene for hypodontia. Segregation analysis will be performed by Haldane's method to determine the modes on inheritance. If necessary, other methods of segregation analysis such as maximum likelihood to fit a major gene model will be used. In those families which satisfy three of the four minimal prerequisites for classical linkage analysis, the following polymorphic system will be typed; red cell antigens, Gm and Imv of the immunoglobulin systems, genetic markers in the plasma, and red cell enzymes; and hemoglobin electrophoresis. Saliva samples will be collected for determining secretor status. Linkage analysis will be done by the method of maximum likelihood uring programs LIPED and GENPED, which allow for incomplete penetrance. Analysis will be done separately by sex of the informative parent if they is any suggestion of linkage. The proposed study is important for several reasons. If one could predict that an individual at risk for a specific from of hypodontia would lack certain teeth, then one might be able to provide preventive orthodontic care. If close linkage were firmly established between some serious systemic disease and one or more of the forms of hyprodontia, the dental abnormalities could be used as advantageous diagnotic markers for the more serious disorder. The results of the study will form the basis for studies of the underlying biochemical and developmental mechanism which produce the disorders.